The recycling industry engages in the selective recovery of polymer materials, for example such as polyethylene (PE), polypropylene (PP), acrylonitrile-butadiene-styrene (ABS), polystyrene (PS), polyvinyl chloride (PVC), polyurethanes (PU) and others. The present invention relates to methods and a corresponding plant which ensure the upgrading of spent polymer materials originating from all types of wastes, and more particularly from motor vehicles. It is hence a matter of achieving a very selective separation of mixtures of polymer materials according to their components, and also the conversion thereof into materials, which can be reused or reintroduced into a new cycle of manufacture.
The sorting of polymer materials can be achieved in several ways, such as by manual sorting, sorting after cryogenic grinding, sorting with electrostatic processes, sorting by infrared analysis or by laser radiation, density sorting, and sorting according to color and shapes. Interest is shown more particularly here in the sorting of large amounts of polymer materials of different natures.
A plant for recycling polymer materials is described in DE-A-4,329,270, and comprises, in particular, a grinder, a discharge basin for removing the heaviest materials, an air-flow separator for separating the polymers in the form of sheets, and then density separators. In the discharge basin, polymer materials with a density greater than 1 g/cm3 can be removed, however, there is no cleaning apparatus for polymer materials. The inclusion of a cleaning apparatus proves to be essential in the case of wastes originating from public garbage dumps or from motor vehicle breakers.
U.S. Pat. No. 4,728,045 describes a method for recovering synthetic materials originating from bottles of polymer materials. A grinding step, an air-flow separation in order to remove light materials such as paper and PP in film form, two flotation separations in medium of a specific density in order to separate the PE from the polyethylene-terephthalate (PET), and a further aerodynamic separation are successively performed. This method is applicable only to bottles of polymer materials and other scrap articles whose polymer composition is precisely known at the beginning of the reclaiming process.
A method for separating polymer materials originating from containers is described in WO-A-92/22,380. This method comprises a grinding step, an airflow separation step, a material cleaning step, and several density separation steps. This method is applicable only to one type of starting substrate whose polymer composition is precisely known.
Document Kunststoffberater, 38, June 1993, No. 6, pages 26 to 30, describes a plant comprising a grinder, a washer and density separators. The washer has a rotating-drum device. However, the starting substrate contains from 95 to 100% PVC and from 0 to 5% of impurities. This plant is only designed for the separation of PVCs.
Document Kunststoffe 80, April 1990, No. 4, pages 493 to 495, describes a plant for separating polymer materials generally like that described above, comprising a grinder, a cleaning device in the form of a rotating drum with nozzles, and density separators.
A process for separating polymer materials is described in FR-A-2,599,279. After grinding and washing, there is a centrifugation in a hydrocyclone, then sorting with a shaking screen.
AT-363,051 describes a process for recovering synthetic materials, which consists of a grinding step, a first flotation step, washing and a further grinding step, followed directly by a second flotation operation. These methods give insufficient results in terms of quality of the polymer materials obtained after separation.
None of these cited methods give satisfactory results in separating polymer materials from waste streams. They are slow or require a very large initial investment. They are also inapplicable to sorting at high rate and therefore cannot be adapted to industrial scale.
Thus, it appears that no separation process of the prior art can be used for sorting mixtures of a wide variety of polymer materials originating from the grinding of motor vehicles or from other sources of wastes.